Methylbutynol (2-methyl-3-butyn-2-ol; C5H8O; CAS-Nr. 115-19-5), is an important intermediate in organic synthesis. For example, it is used in the production of isoprene, vitamins A and E, various pharmaceuticals and aroma compounds. On an industrial scale, methylbutynol is usually produced by reacting acetone with acetylene. Thereby, a crude reaction product is obtained, which comprises significant amounts of water and unreacted acetone. The reaction is carried out in the presence of catalysts and in specific solvents, such as liquid ammonia, acetals or ethers, which are also components of the crude reaction product. Further, salts and low-boiling organic by-products may be present in the crude reaction product.
In order to obtain pure methylbutynol it is necessary to remove water, acetone and further ingredients from the crude reaction product. However, removal of water is difficult because the boiling point of water is in the same range as the boiling point of methylbutynol (104° C.). Further, water and methylbutynol form an azeotrope boiling at about 91° C., consisting of about 70.5 to 72.5% (w/w) methylbutynol and 27.5 to 29.5% (w/w) water. Thus, water removal by normal distillation is not possible.
Acetone, which may be present in the crude reaction product in amounts of about 0.5 to 10% (w/w), has a boiling point of 56° C. and is removable by distillation. However, important reactions, such as polymerization of isoprene with Ziegler catalysts, are inhibited by trace amounts of acetone. Therefore, for many applications of methylbutynol it is necessary to remove acetone from the methylbutynol almost quantitatively, for example to a level below 0.03% (w/w).
According to prior art, the crude reaction product of the reaction of acetone and acetylene comprising the desired methylbutynol is usually subjected to azeotropic distillation in the presence of an entrainer forming an azeotrope with water. The entrainer/water azeotrope is removed from the reaction mixture by distillation. Low-boiling acetone is also removed with the distillate while methylbutynol containing low residual amounts of water and acetone remains at the bottom of the distillation column.
A common entrainer in this prior art process is benzene. Methods for purifying methylbutynol by distillation in the presence of benzene are disclosed in DE 1 193 496 and GB 1,023,856. Nowadays, the use of benzene is no longer considered acceptable in industrial processes for health and ecological considerations.
It is another drawback of such processes that a water/entrainer/methylbutynol ternary azeotrope containing a relatively high concentration of methylbutynol is formed. Thus either the overall yield is decreased, or additional steps for recovering methylbutynol from the azeotrope are required.
The introduction of entrainers into the distillation process has further drawbacks. Generally, entrainers render the overall process more complicated. Entrainers are found in the product and specific conditions and additional process steps have to be selected to keep the concentration of the entrainer in the methylbutynol product as low as possible. Further, the entrainer increases the total amount of the overall feed composition. Thus, the overall distillation process, which is carried out at about 100 ° C., requires more energy for evaporation and condensation of the components. Further energy and additional process steps are required for recycling the entrainer, i. e. separating it from water and methylbutynol. The use of an entrainer thus increases overall waste, emissions and complexity of the process, especially when carried out as a continuous process.
Problem Underlying the Invention
The problem underlying the invention is to provide an improved process for the purification of methylbutynol, which overcomes the above-mentioned drawbacks. The process shall be relatively simple, cost- and energy-efficient. Specifically, a process shall be provided to obtain methylbutynol from a crude reaction product comprising acetone and water. The obtainable methylbutynol product shall contain very low amounts of water, acetone and other side-products. The use of environmentally harmful substances, such as benzene or other aromatic compounds, shall be avoided.
It is a specific problem underlying the invention to provide a process which avoids the introduction of an entrainer. The process shall be applicable as a continuous process and in large industrial scale for the purification of large amounts of methylbutynol.